Laser sources are important to data transmission in communication networks, small and large. In smaller-scale networks like local-area networks (LANs), for example, an individual laser source may be used to produce a signal at a single frequency. Modulation of that single frequency imparts the data for that signal. For larger networks such as metro-area networks (MANs), a wavelength division multiplexing (WDM) system may employ a laser source capable of producing a range of output frequencies. In this latter example, many channels that each represents a different data stream may be propagated on a single optical fiber. An example dense WDM system might include approximately one-hundred channels propagating on a single fiber, each channel emitting from a laser source capable of producing a range of output frequencies within the C-band from 1525 to 1565 nm.
WDM laser sources may include banks of individual lasers, each producing a different channel. The output signals from these individual lasers are typically multiplexed together into an optical fiber or fiber bundle. The WDM laser sources may be tunable lasers or “single” frequency lasers.
In these and many other applications, stable control over laser source output frequency and bandwidth is paramount to device operation. In fact, there are numerous networking industry standards that set-forth acceptable laser transponder performance. Various Telcordia™ Technology's, Synchronous Optical Network's (SONET), and International Telecommunication Union's (ITU) standards are examples, some of which are directed to wavelength control.
Although communications standards are in place and fabrication techniques well-developed, lasers designed for stability and predictability can in fact produce inaccurate output frequencies over the device lifetime (a lifetime often specified as 25 years). These lasers may fail to produce acceptable output bandwidths and power levels, as well.
To correct for these and other problems, wavelength locking techniques are often used. The principle desire behind wavelength locking is a need for stable and predictable output frequency. Wavelength locking is commonly achieved by a wavelength reference filter, or wavelength locker. Most wavelength reference filters not only optimize output frequency, but also they simultaneously produce narrow bandwidth output signals when used with appropriate servo systems. With such benefits, wavelength reference devices are particularly useful in WDM systems where frequency control and tight channel spacing between frequencies is essential to avoid crosstalk errors. Wavelength reference filters may both correct and enhance laser source performance.
There are numerous ways to design a laser with precise output frequency and bandwidth. A relatively affordable, commonly used technique for wavelength locking is to use a partially-transmitting, resonant-cavity filtering element, such as an etalon. The etalon is an optical device that is only able to sustain wavelengths that are harmonics of its cavity length. The sustainable output wavelengths from an etalon are set by the resonance condition of the etalon cavity, and, as a result, the etalon may be used to determine the accuracy to which an input wavelength matches a desired output wavelength, by selecting an etalon with a predetermined optical cavity length. Etalons are tunable across a range of optical cavity lengths, for example through temperature tuning.
Although useful, etalons may introduce a relatively low but nevertheless detrimental error to laser source operation. Some of the problems stem from the physical properties of an etalon. For example, etalons are partially transmissive and partially reflective. When an etalon is placed perpendicular to an incident laser signal (an orientation that optimizes etalon transmission), etalons may reflect substantial amounts of the laser energy back at the laser source. For this reason, etalons are typically tilted from this perpendicular or normal incidence. Such an etalon tilt, however, introduces another problem—a first order dependence of output frequency on the angle of incidence of the laser energy. As a result of this dependence, perturbation of the incidence angle will result in an error in the output frequency from the etalon. For small tilting angles, the error may be tolerably low for coarse wavelength division multiplexing networks. Dense wavelength division multiplexing networks utilize ever increasing numbers of channels, with concomitantly ever decreasing tolerances for crosstalk-inducing wavelength error. These same etalon deficiencies plague independent detection systems, as well as integrated laser sources. Detection systems based on etalons may have reduced performance, as a result.